1. Field of the Invention
The present invention relates to a method for forming a phase hologram. Particularly, it relates to a method for forming a phase hologram by bleaching an amplitude hologram recorded on a silver halide photographic material.
2. Description of the Prior Art
An amplitude hologram (represented by an amplitude transmittance distribution) is obtained by exposing a silver halide photographic material to an interference fringe formed by coherent light beams, developing and fixing the photographic material to form a silver image (interference fringe). The silver which comprises the amplitude hologram can be converted into a transparent silver compound by bleaching to produce a phase hologram in which the refractive index of the silver compound differs from the refractive index of the binder.
Since an amplitude hologram represents an image-wise distribution of amplitude transmittance, the brightness of a reconstructed image from an amplitude hologram is small as compared to a phase hologram. One of the principal factors which influences the characteristics of a hologram is the diffraction efficiency (represented by .eta.,.eta. = (I.sub.1 I).times. 100 (%), wherein I designates incident light power and I.sub.1 the transmission first order diffraction light power). A phase hologram, which provides a bright reconstructed image, provides a much higher .eta. value as compared with an amplitude hologram.
Heretofore, attempts have been made to bleach holograms using ammonium dichromate, potassium ferricyanide, mercuric chloride, or copper bromide, as are used in conventional photographic processing, as described in Applied Optics, Vol. 7, p. 2409 (1968), Vol. 8, p. 85 (1969) and Vol. 9, p. 1363 (1970).
Although phase holograms produced using conventional bleaching treatments provide a high diffraction efficiency, the light resistance thereof is poor and silver compounds of most of these holograms are blackened (printed-out) by the light used for reconstruction.
Of these conventional treatments, the treatment wherein the silver developed is converted into silver iodide has been considered to provide a phase hologram having relatively good light resistance (for example, bleaching treatment R-10, developed by Eastman Kodak Co.: a bleaching treatment using a solution prepared by mixing an aqueous solution of sulfuric acid containing ammonium dichromate and an aqueous solution of potassium iodide). However, with this method, the silver developed is not completely converted into silver iodide, and impurities are likely to act as nuclei which cause print-out and blacken bleached areas. Further, when the above-described silver compound is converted into silver iodide, crystal growth occurs and the resultant hologram has the disadvantage that the light scattering thereof is increased.